Polymer Membranes Prepared from Aromatic Polyimide Membranes by Thermal Treating and UV Crosslinking

ABSTRACT

The present invention discloses a new type of high performance polymer membranes prepared from aromatic polyimide membranes by thermal treating and crosslinking and methods for making and using these membranes. The polymer membranes were prepared from aromatic polyimide membranes by thermal treating under inert atmosphere followed by crosslinking preferably by using a UV radiation source. The aromatic polyimide membranes were made from aromatic polyimide polymers comprising both pendent hydroxy functional groups ortho to the heterocyclic imide nitrogen and cross-linkable functional groups in the polymer backbone. The membranes showed significantly improved selectivity and permeability for gas separations compared to the aromatic polyimide membranes without any treatment. The membranes can be fabricated into any convenient geometry and are not only suitable for a variety of liquid, gas, and vapor separations, but also can be used for other applications such as for catalysis and fuel cell applications.

BACKGROUND OF THE INVENTION

This invention pertains to a new type of high performance polymermembranes prepared from aromatic polyimide membranes by thermal treatingand UV crosslinking and methods for making and using these membranes.

In the past 30-35 years, the state of the art of polymer membrane-basedgas separation processes has evolved rapidly. Membrane-basedtechnologies have advantages of both low capital cost and high-energyefficiency compared to conventional separation methods. Membrane gasseparation is of special interest to petroleum producers and refiners,chemical companies, and industrial gas suppliers. Several applicationshave achieved commercial success, including carbon dioxide removal fromnatural gas and from biogas and enhanced oil recovery, and also inhydrogen removal from nitrogen, methane, and argon in ammonia purge gasstreams. For example, UOP's Separex™ cellulose acetate polymericmembrane is currently an international market leader for carbon dioxideremoval from natural gas.

The membranes most commonly used in commercial gas separationapplications are polymeric and nonporous. Separation is based on asolution-diffusion mechanism. This mechanism involves molecular-scaleinteractions of the permeating gas with the membrane polymer. Themechanism assumes that in a membrane having two opposing surfaces, eachcomponent is sorbed by the membrane at one surface, transported by a gasconcentration gradient, and desorbed at the opposing surface. Accordingto this solution-diffusion model, the membrane performance in separatinga given pair of gases (e.g., CO₂/CH₄, O₂/N₂, H₂/CH₄) is determined bytwo parameters: the permeability coefficient (abbreviated hereinafter asP_(A)) and the selectivity (α_(A/B)). The P_(A) is the product of thegas flux and the selective skin layer thickness of the membrane, dividedby the pressure difference across the membrane. The α_(A/B) is the ratioof the permeability coefficients of the two gases (α_(A/B)=P_(A)/P_(B))where P_(A) is the permeability of the more permeable gas and P_(B) isthe permeability of the less permeable gas. Gases can have highpermeability coefficients because of a high solubility coefficient, ahigh diffusion coefficient, or because both coefficients are high. Ingeneral, the diffusion coefficient decreases while the solubilitycoefficient increases with an increase in the molecular size of the gas.In high performance polymer membranes, both high permeability andselectivity are desirable because higher permeability decreases the sizeof the membrane area required to treat a given volume of gas, therebydecreasing capital cost of membrane units, and because higherselectivity results in a higher purity product gas.

Polymers provide a range of properties including low cost, permeability,mechanical stability, and ease of processability that are important forgas separation. A polymer material with a high glass-transitiontemperature (T_(g)), high melting point, and high crystallinity ispreferred. Glassy polymers (i.e., polymers at temperatures below theirT_(g)) have stiffer polymer backbones and therefore let smallermolecules such as hydrogen and helium pass through more quickly, whilelarger molecules such as hydrocarbons pass through more slowly ascompared to polymers with less stiff backbones. However, polymers whichare more permeable are generally less selective than are less permeablepolymers. A general trade-off has always existed between permeabilityand selectivity (the so-called polymer upper bound limit). Over the past30 years, substantial research effort has been directed to overcomingthe limits imposed by this upper bound. Various polymers and techniqueshave been used, but without much success. In addition, traditionalpolymer membranes also have limitations in terms of thermal stabilityand contaminant resistance.

Cellulose acetate (CA) glassy polymer membranes are used extensively ingas separation. Currently, such CA membranes are used for natural gasupgrading, including the removal of carbon dioxide. Although CAmembranes have many advantages, they are limited in a number ofproperties including selectivity, permeability, and in chemical,thermal, and mechanical stability. For example, it has been found inpractice that polymer membrane performance can deteriorate quickly. Theprimary cause of loss of membrane performance is liquid condensation onthe membrane surface. Condensation is prevented by providing asufficient dew point margin for operation, based on the calculated dewpoint of the membrane product gas. UOP's MemGuard™ system, apretreatment regenerable adsorbent system that uses molecular sieves,was developed to remove water as well as heavy hydrocarbons ranging fromC₁₀ to C₃₅ from the natural gas stream, hence, to lower the dew point ofthe stream. The selective removal of heavy hydrocarbons by apretreatment system can significantly improve the performance of themembranes. Although these pretreatment systems can effectively removeheavy hydrocarbons from natural gas streams to lower their dew point,the cost is quite significant. Some projects showed that the cost of thepretreatment system was as high as 10 to 40% of the total cost(pretreatment system and membrane system) depending on the feedcomposition. Reduction of the pretreatment system cost or totalelimination of the pretreatment system would significantly reduce themembrane system cost for natural gas upgrading. On the other hand, inrecent years, more and more membrane systems have been applied to largeoffshore natural gas upgrading projects. For offshore projects, thefootprint is a big constraint. Hence, reduction of footprint is veryimportant for offshore projects. The footprint of the pretreatmentsystem is also very high at more than 10-50% of the footprint of thewhole membrane system. Removal of the pretreatment system from themembrane system has great economical impact especially to offshoreprojects.

High performance polymers such as polyimides (PIs),poly(trimethylsilylpropyne) (PTMSP), and polytriazole have beendeveloped recently to improve membrane selectivity, permeability, andthermal stability. These polymeric membrane materials have shownpromising properties for separation of gas pairs such as CO₂/CH₄, O₂/N₂,H₂/CH₄, and propylene/propane (C₃H₆/C₃H₈). These high performancepolymeric membrane materials, however, have reached a limit in theirpermeability-selectivity trade-off relationship. The membranes havinghigh permeabilities generally have low selectivities and vice versa. Inaddition, gas separation processes based on the use of glassysolution-diffusion membranes frequently suffer from plasticization ofthe stiff polymer matrix by the sorbed penetrant molecules such as CO₂or C₃H₆. Plasticization of the polymer as represented by the membranestructure swelling and significant increase in the permeabilities of allcomponents in the feed occurs above the plasticization pressure when thefeed gas mixture contains condensable gases.

Barsema et al. reported that heat treatment of Matrimid® membranes in aninert atmosphere can alter the membrane properties as well as molecularstructure. See Barsema, et al., J. MEMBR. SCI., 238: 93 (2004). Theseheat-treated polyimide membranes showed improved plasticizationresistance. However, these heat-treated polyimide membranes did not showsignificant improvement in selectivity and permeability compared to theuntreated polyimide membranes.

In US 2008/0300336 A1, it was reported that the use of UV crosslinkingdid succeed in improving the selectivities of certain mixed matrixmembranes that contain molecular sieves that function to improve thepermeability and selectivity of the membranes. However, it was necessaryto both crosslink the polymer and to add the molecular sieves to obtainthe improved levels of performance reported therein. None of themembranes reported in US 2008/0300336 A1 exhibited CO₂ permeabilityhigher than 200 Barrer and CO₂/CH₄ selectivity over 40 at 50° C. testingtemperature for the removal of CO₂ from natural gas. It is highlydesired to have improved polymeric membranes that do not containmolecular sieves both to avoid the need to disperse the molecular sievesand to eliminate any problems caused by the lack of adhesion between thepolymer and the molecular sieves.

Therefore, a new polymer membrane possessing both high permeability andhigh selectivity is still needed.

The present invention provide a new type of high performance polymer tomembranes overcoming the problems of the prior art polymer membranes.These new polymer membranes have both high selectivity and highpermeability (or permeance), as well as high thermal stability.

SUMMARY OF THE INVENTION

This invention pertains to a new type of high performance polymermembranes prepared from aromatic polyimide membranes by thermal treatingand UV crosslinking and methods for making and using these membranes.

The high performance polymer membranes described in the currentinvention are prepared from aromatic polyimide membranes by thermaltreating under inert atmosphere (e.g., nitrogen, argon or vacuum)followed by UV crosslinking using a UV radiation source. The aromaticpolyimide membranes described in the current invention were made fromaromatic polyimide polymers comprising both UV cross-linkable functionalgroups in the polymer backbone and pendent hydroxy functional groupsortho to the heterocyclic imide nitrogen. The novel high performancepolymer membranes prepared from aromatic polyimide membranes by thermaltreating and UV crosslinking showed significantly improved selectivityand permeability for gas separations compared to the aromatic polyimidemembranes without any treatment. It is believed that the improvementobtained in both selectivity and permeability after thermal treating andUV crosslinking is not only related to the reaction between theheterocyclic imide groups and the pendent hydroxy groups ortho to theheterocyclic imide nitrogen, but also related to the formation of threedimensional crosslinked network structures due to the crosslinking ofthe polymer chain segments to each other through possible directcovalent bonds.

The high performance polymer membranes of the present invention overcomethe problems of the prior art polymer membranes with the advantages ofhigh selectivity, high permeability (or permeation), high thermalstability, and stable flux and sustained selectivity over time byresistance to solvent swelling, plasticization and hydrocarboncontaminants.

The present invention provides a method for the production of the highperformance polymer membrane by: 1) preparing an aromatic polyimidepolymer membrane from an aromatic polyimide polymer comprising pendenthydroxy groups ortho to the heterocyclic imide nitrogen and UVcrosslinkable functional groups (e.g., carbonyl group) in the polymerbackbone; 2) thermal treating the aromatic polyimide polymer membrane byheating between 300° and 600° C. under inert atmosphere, such as argon,nitrogen, or vacuum; and 3) UV crosslinking the thermal-treated aromaticpolyimide polymer membrane from step 2) by UV radiation. In some cases amembrane post-treatment step can be added after step 3) by coating theselective layer surface of the both thermal-treated and UV-treatedaromatic polyimide polymer membrane with a thin layer of highpermeability material such as a polysiloxane, a fluoro-polymer, athermally curable silicone rubber, or a UV radiation curable epoxysilicone.

The new high performance polymer membranes prepared from aromaticpolyimide membranes by thermal treating and UV crosslinking in thepresent invention can have either a nonporous symmetric structure or anasymmetric structure with a thin nonporous dense selective layersupported on top of a porous support layer. The new high performancepolymer membranes of the present invention be fabricated into anyconvenient geometry such as flat sheet (or spiral wound), disk, tube,hollow fiber, or thin film composite.

The invention provides a process for separating at least one gas orliquid from a mixture of gases or liquids using the polymer membranesprepared from aromatic polyimide membranes by thermal treating and UVcrosslinking described in the present invention, the process comprising:(a) providing a polymer membrane prepared from an aromatic polyimidemembrane by thermal treating and UV crosslinking which is permeable tosaid at least one gas or liquid; (b) contacting the mixture on one sideof the polymer membrane prepared from the aromatic polyimide membrane bythermal treating and UV crosslinking to cause said at least one gas orliquid to permeate the membrane; and (c) removing from the opposite sideof the membrane a permeate gas or liquid composition comprising aportion of said at least one gas or liquid which permeated saidmembrane.

The novel high performance polymer membranes prepared from aromaticpolyimide membranes by thermal treating and UV crosslinking showeddramatically improved selectivities and permeabilities for a wide rangeof separations such as for CO₂/CH₄, H₂/CH₄, O₂/N₂ and propylene/propaneseparations. For example, the new polymer membrane prepared from thermaltreating and UV crosslinking of thepoly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(BTDA-APAF)) polyimide membrane showed CO₂ permeability (P_(CO2))of 220 Barrer and CO₂/CH₄ selectivity (α_(CO2)/CH₄) of 48.4 for CO₂/CH₄separation compared to the untreated poly(BTDA-APAF)) polyimide membranewith P_(CO2) of 5.92 Barrer and α_(CO2/CH4) of 32.5.

The new high performance polymer membranes of the present invention arenot only suitable for a variety of liquid, gas, and vapor separationssuch as desalination of water by reverse osmosis, non-aqueous liquidseparation such as deep desulfurization of gasoline and diesel fuels,ethanol/water separations, pervaporation dehydration of aqueous/organicmixtures, CO₂/CH₄, CO₂/N₂, H₂/CH₄, O₂/N₂, H₂S/CH₄, olefin/paraffin,iso/normal paraffins separations, and other light gas mixtureseparations, but also can be used for other applications such as forcatalysis and fuel cell applications.

DETAILED DESCRIPTION OF THE INVENTION

The present invention involves a new type of high performance polymermembranes prepared from aromatic polyimide membranes by thermal treatingand UV crosslinking and methods for making and using these membranes.

The high performance polymer membranes of the present invention overcomethe problems of the prior art polymer membranes with the advantages ofhigh selectivity, high permeability (or permeation), high thermalstability, and stable flux and sustained selectivity over time byresistance to solvent swelling, plasticization and hydrocarboncontaminants.

The high performance polymer membranes described in the currentinvention are prepared from aromatic polyimide membranes by thermaltreating followed by UV crosslinking. The aromatic polyimide membranesdescribed in the current invention were prepared from aromatic polyimidepolymers comprising both UV crosslinkable functional groups such asbenzophenone group in the polymer backbone and pendent hydroxyfunctional groups ortho to the heterocyclic imide nitrogen. The thermaltreatment and UV cross-linking offer the aromatic polyimide membranessignificantly improved selectivity, permeability, as well as chemicaland thermal stabilities compared to the untreated aromatic polyimidemembranes. It is believed that the performance improvement after thermaltreating and UV crosslinking is not only related to the reaction betweenthe heterocyclic imide groups and the pendent hydroxy groups ortho tothe heterocyclic imide nitrogen, but also related to the formation ofthree dimensional crosslinked network structure due to the crosslinkingof the polymer chain segments to each other through possible directcovalent bonds.

The present invention provides a method for the production of the highperformance polymer membrane by: 1) preparing an aromatic polyimidepolymer membrane to from an aromatic polyimide polymer comprisingpendent hydroxy groups ortho to the heterocyclic imide nitrogen and UVcrosslinkable functional groups (e.g., carbonyl group) in the polymerbackbone; 2) thermal treating the aromatic polyimide polymer membrane;and 3) UV crosslinking the thermal-treated aromatic polyimide polymermembrane from step 2). In some cases a membrane post-treatment step canbe added after step 3) by coating the selective layer surface of theboth thermal-treated and UV-treated aromatic polyimide polymer membranewith a thin layer of high permeability material such as a polysiloxane,a fluoro-polymer, a thermally curable silicone rubber, or a UV radiationcurable epoxy silicone.

The thermal treatment for the aromatic polyimide polymer membranes isconducted by heating the membrane between 300° and 600° C. under inertatmosphere, such as argon, nitrogen, or vacuum. It is proposed thatthere is an irreversible molecular rearrangement reaction between theheterocyclic imide groups and the pendent hydroxy groups ortho to theheterocyclic imide nitrogen during the thermal treatment process. The UVcrosslinking of the thermal-treated aromatic polyimide polymer membranesis done by irradiating the membrane with a UV radiation source. It isbelieved that this UV crosslinking step results in the formation ofthree dimensional crosslinked network structures due to the crosslinkingof the polymer chain segments to each other through possible directcovalent bonds.

The aromatic polyimide polymers comprising both UV crosslinkablefunctional groups and pendent hydroxy functional groups that are usedfor the preparation of the new high performance polymer membranes in thepresent invention comprise a plurality of first repeating units of aformula (I), wherein said formula (I) is:

where —X₁— of said formula (I) is

or mixtures thereof, —X₂— of said formula (I) is either the same as —X₁—or is selected from

or mixtures thereof, —X₃— of said formula (I) is

or mixtures thereof, —R— is

or mixtures thereof.

Some preferred aromatic polyimide polymers comprising both UVcrosslinkable functional groups and pendent hydroxy functional groupsthat are used for the preparation of the new high performance polymermembranes in the present invention include, but are not limited to,poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(BTDA-APAF)), poly[4,4′-oxydiphthalicanhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(ODPA-APAF)), poly(3,3′,4,4′-benzophenonetetracarboxylicdianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl) (poly(BTDA-HAB)),poly[3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(DSDA-APAF)), poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl)(poly(DSDA-APAF-HAB)),poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(6FDA-BTDA-APAF)), poly[4,4′-oxydiphthalicanhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl](poly(ODPA-APAF-HAB)), poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl(poly(BTDA-APAF-HAB)), and poly(4,4′-bisphenol Adianhydride-3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(BPADA-BTDA-APAF)).

The aromatic polyimide polymers comprising both UV crosslinkablefunctional groups and pendent hydroxy functional groups that are usedfor the preparation of the new high performance polymer membranes in thepresent invention are synthesized from diamine monomers and dianhydridemonomers in polar solvents such as 1-methyl-2-pyrrolidione (NMP) orN,N-dimethylacetamide (DMAc) by a two-step process involving theformation of the poly(amic acid)s followed by a solution imidization ora thermal imidization. Acetic anhydride is used as the dehydrating agentand pyridine (or triethylamine) is used as the imidization catalyst forthe solution imidization reaction.

The aromatic polyimide membrane in the present invention can befabricated into a membrane with nonporous symmetric thin film geometryfrom the aromatic polyimide polymer comprising UV cross-linkablefunctional groups and pendent hydroxy functional groups ortho to theheterocyclic imide nitrogen in the polymer backbone by casting ahomogeneous aromatic polyimide solution on top of a clean glass plateand allowing the solvent to evaporate slowly inside a plastic cover forat least 12 hours at room temperature. The membrane is then detachedfrom the glass plate and dried at room temperature for 24 hours and thenat 200° C. for at least 48 hours under vacuum.

The aromatic polyimide membrane in the present invention can also befabricated by a method comprising the steps of: dissolving the aromaticpolyimide polymer in a solvent to form a solution of the polyimidematerial; contacting a porous membrane support (e.g., a support madefrom inorganic ceramic material) with said solution; and evaporating thesolvent to provide a thin selective layer comprising the aromaticpolyimide polymer material on the supporting layer.

The aromatic polyimide membrane in the present invention can also befabricated as an asymmetric membrane with flat sheet or hollow fibergeometry by phase inversion followed by direct air drying through theuse of at least one drying agent which is a hydrophobic organic compoundsuch as a hydrocarbon or an ether (see U.S. Pat. No. 4,855,048). Thearomatic polyimide membrane in the present invention can also befabricated as an asymmetric membrane with flat sheet or hollow fibergeometry by phase inversion followed by solvent exchange methods (seeU.S. Pat. No. 3,133,132).

The solvents used for dissolving the aromatic polyimide polymercomprising both UV crosslinkable functional groups and pendent hydroxyfunctional groups are chosen primarily for their ability to completelydissolve the polymers and for ease of solvent removal in the membraneformation steps. Other considerations in the selection of solventsinclude low toxicity, low corrosive activity, low environmental hazardpotential, availability and cost. Representative solvents for use inthis invention include most amide solvents that are typically used forthe formation of polymeric membranes, such as N-methylpyrrolidone (NMP)and N,N-dimethyl acetamide (DMAC), methylene chloride, tetrahydrofuran(THF), acetone, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO),toluene, dioxanes, 1,3-dioxolane, mixtures thereof, others known tothose skilled in the art and mixtures thereof.

The aromatic polyimide polymer membrane in the present invention wasthermally treated between 300° and 600° C. under inert atmosphere, suchas argon, nitrogen, or vacuum. The thermally treated aromatic polyimidepolymer membrane was then further UV treated to crosslink the membraneby irradiating the membrane with a UV radiation source. One method to dothe UV treatment is to use a UV lamp from a predetermined distance andfor a period of time selected based upon the separation propertiessought. For example, the thermally treated aromatic polyimide polymermembrane can be further UV treated by exposure to UV radiation using 254nm wavelength UV light generated from a UV lamp with 1.9 cm (0.75 inch)distance from the membrane surface to the UV lamp and a radiation timeof 30 minutes at less than 50° C. The UV lamp described here is a lowpressure, mercury arc immersion UV quartz 12 watt lamp with 12 wattpower supply from Ace Glass Incorporated. Optimization of the conditionsfor the UV treatment should promote the tailoring of the membranes for awide range of gas and liquid separations with improved permeationproperties and environmental stability. The UV cross-linking degree ofthe thermally treated aromatic polyimide polymer membrane can becontrolled by adjusting the distance between the UV lamp and themembrane surface, UV radiation time, wavelength and strength of UVlight, etc. Preferably, the distance from the UV lamp to the membranesurface is in the range of 0.8 to 25.4 cm (0.3 to 10 inches) with a UVlight provided from 12 watt to 450 watt low pressure or medium pressuremercury arc lamp, and the UV radiation time is in the range of 0.5minute to 1 hour. More preferably, the distance from the UV lamp to themembrane surface is in the range of 1.3 to 5.1 cm (0.5 to 2 inches) witha UV light provided from 12 watt to 450 watt low pressure or mediumpressure mercury arc lamp, and the UV radiation time is in the range of0.5 to 40 minutes.

In some cases a membrane post-treatment step can be added after thethermal treating and UV crosslinking steps by introducing a thin layerof high permeability material such as a polysiloxane, a fluoro-polymer,a thermally curable silicone rubber, or a UV radiation curable epoxysilicone. The coating filling the surface pores and other imperfectionscomprising voids (see U.S. Pat. No. 4,230,463; U.S. Pat. No. 4,877,528;U.S. Pat. No. 6,368,382).

The new high performance polymer membranes prepared from the aromaticpolyimide membranes by thermal treating and UV crosslinking described inthe present invention can have either a nonporous symmetric structure oran asymmetric structure with a thin nonporous dense selective layersupported on top of a porous support layer. The porous support can bemade from the same aromatic polyimide material or a different type ofmaterial with high thermal stability. The new high performance polymermembranes of the present invention can be fabricated into any convenientgeometry such as flat sheet (or spiral wound), disk, tube, hollow fiber,or thin film composite.

The invention provides a process for separating at least one gas orliquid from a mixture of gases or liquids using the new polymermembranes prepared from the aromatic polyimide membranes by thermaltreating and UV crosslinking described in the present invention, theprocess comprising: (a) providing the new polymer membrane prepared fromthe aromatic polyimide membrane by thermal treating and UV crosslinkingwhich is permeable to said at least one gas or liquid; (b) contactingthe mixture on one side of the new polymer membrane prepared from thearomatic polyimide membrane by thermal treating and UV crosslinking tocause said at least one gas or liquid to permeate the membrane; and (c)removing from the opposite side of the membrane a permeate gas or liquidcomposition comprising a portion of said at least one gas or liquidwhich permeated said membrane.

The new polymer membranes prepared from the aromatic polyimide membranesby thermal treating and UV crosslinking of the present invention areespecially useful in the purification, separation or adsorption of aparticular species in the liquid or gas phase. In addition to separationof pairs of gases, these high performance polymer membranes may, forexample, can be used for the desalination of water by reverse osmosis orfor the separation of proteins or other thermally unstable compounds,e.g. in the pharmaceutical and biotechnology industries. The new polymermembranes prepared from the aromatic polyimide membranes by thermaltreating and UV crosslinking described in the present invention may alsobe used in fermenters and bioreactors to transport gases into thereaction vessel and transfer cell culture medium out of the vessel.Additionally, these new polymer membranes may be used for the removal ofmicroorganisms from air or water streams, water purification, ethanolproduction in a continuous fermentation/membrane pervaporation system,and in detection or removal of trace compounds or metal salts in air orwater streams.

The new polymer membranes prepared from the aromatic polyimide membranesby thermal treating and UV crosslinking described in the presentinvention are especially useful in gas separation processes in airpurification, petrochemical, refinery, and natural gas industries.Examples of such separations include separation of volatile organiccompounds (such as toluene, xylene, and acetone) from an atmosphericgas, such as nitrogen or oxygen and nitrogen recovery from air. Furtherexamples of such separations are for the separation of CO₂ or H₂S fromnatural gas, H₂ from N₂, CH₄, and Ar in ammonia purge gas streams, H₂recovery in refineries, olefin/paraffin separations such aspropylene/propane separation, and iso/normal paraffin separations. Anygiven pair or group of gases that differ in molecular size, for examplenitrogen and oxygen, carbon dioxide and methane, hydrogen and methane orcarbon monoxide, helium and methane, can be separated using the newpolymer membranes prepared from the aromatic polyimide membranes bythermal treating and UV crosslinking described herein. More than twogases can be removed from a third gas. For example, some of the gascomponents which can be selectively removed from a raw natural gas usingthe membrane described herein include carbon dioxide, oxygen, nitrogen,water vapor, hydrogen sulfide, helium, and other trace gases. Some ofthe gas components that can be selectively retained include hydrocarbongases. When permeable components are acid components selected from thegroup consisting of carbon dioxide, hydrogen sulfide, and mixturesthereof and are removed from a hydrocarbon mixture such as natural gas,one module, or at least two in parallel service, or a series of modulesmay be utilized to remove the acid components. For example, when onemodule is utilized, the pressure of the feed gas may vary from 275 kPato about 2.6 MPa (25 to 4000 psi). The differential pressure across themembrane can be as low as about 0.7 bar or as high as 145 bar (about 10psi or as high as about 2100 psi) depending on many factors such as theparticular membrane used, the flow rate of the inlet stream and theavailability of a compressor to compress the permeate stream if suchcompression is desired. Differential pressure greater than about 145 bar(2100 psi) may rupture the membrane. A differential pressure of at least7 bar (100 psi) is preferred since lower differential pressures mayrequire more modules, more time and compression of intermediate productstreams. The operating temperature of the process may vary dependingupon the temperature of the feed stream and upon ambient temperatureconditions. Preferably, the effective operating temperature of themembranes of the present invention will range from about −50° to about150° C. More preferably, the effective operating temperature of themembranes of the present invention will range from about −20° to about100° C., and most preferably, the effective operating temperature of themembranes of the present invention will range from about 25° to about100° C.

The new polymer membranes prepared from the aromatic polyimide membranesby thermal treating and UV crosslinking described in the currentinvention are also especially useful in gas/vapor separation processesin chemical, petrochemical, pharmaceutical and allied industries forremoving organic vapors from gas streams, e.g. in off-gas treatment forrecovery of volatile organic compounds to meet clean air regulations, orwithin process streams in production plants so that valuable compounds(e.g., vinylchloride monomer, propylene) may be recovered. Furtherexamples of gas/vapor separation processes in which these new polymermembranes prepared from the aromatic polyimide membranes by thermaltreating and UV crosslinking may be used are hydrocarbon vaporseparation from hydrogen in oil and gas refineries, for hydrocarbon dewpointing of natural gas (i.e. to decrease the hydrocarbon dew point tobelow the lowest possible export pipeline temperature so that liquidhydrocarbons do not separate in the pipeline), for control of methanenumber in fuel gas for gas engines and gas turbines, and for gasolinerecovery. The new polymer membranes prepared from the aromatic polyimidemembranes by thermal treating and UV crosslinking may incorporate aspecies that adsorbs strongly to certain gases (e.g. cobalt porphyrinsor phthalocyanines for O₂ or silver (I) for ethane) to facilitate theirtransport across the membrane.

The new polymer membranes prepared from the aromatic polyimide membranesby thermal treating and UV crosslinking also have immediate applicationto concentrate olefin in a paraffin/olefin stream for olefin crackingapplication. For example, the new polymer membranes prepared from thearomatic polyimide membranes by thermal treating and UV crosslinking canbe used for propylene/propane separation to increase the concentrationof the effluent in a catalytic dehydrogenation reaction for theproduction of propylene from propane and isobutylene from isobutane.Therefore, the number of stages of propylene/propane splitter that isrequired to get polymer grade propylene can be reduced. Anotherapplication for the new polymer membranes prepared from the aromaticpolyimide membranes by thermal treating and UV crosslinking is forseparating isoparaffin and normal paraffin in light paraffinisomerization and MaxEne™, a process for enhancing the concentration ofnormal paraffin (n-paraffin) in the naphtha cracker feedstock, which canbe then converted to ethylene.

The new polymer membranes prepared from the aromatic polyimide membranesby thermal treating and UV crosslinking can also be operated at hightemperature to provide the sufficient dew point margin for natural gasupgrading (e.g, CO₂ removal from natural gas). The new polymer membraneprepared from the aromatic polyimide membrane by thermal treating and UVcrosslinking can be used in either a single stage membrane or as thefirst or/and second stage membrane in a two stage membrane system fornatural gas upgrading. The new polymer membranes of the presentinvention have high selectivity, high permeance, and high thermal andchemical stabilities that allow the membranes to be operated without acostly pretreatment system. Hence, a costly membrane pretreatment systemsuch as a MemGuard™ system will not be required in the new processcontaining the new polymer membrane system. Due to the elimination ofthe pretreatment system and the significant reduction of membrane area,the new process can achieve significant capital cost saving and reducethe existing membrane footprint.

These new polymer membranes prepared from the aromatic polyimidemembranes by thermal treating and UV crosslinking may also be used inthe separation of liquid mixtures by pervaporation, such as in theremoval of organic compounds (e.g., alcohols, phenols, chlorinatedhydrocarbons, pyridines, ketones) from water such as aqueous effluentsor process fluids. A membrane which is ethanol-selective would be usedto increase the ethanol concentration in relatively dilute ethanolsolutions (5-10% ethanol) obtained by fermentation processes. Anotherliquid phase separation example using these new polymer membranesprepared from the aromatic polyimide membranes by thermal treating andUV crosslinking is the deep desulfurization of gasoline and diesel fuelsby a pervaporation membrane process similar to the process described inU.S. Pat. No. 7,048,846, incorporated by reference herein in itsentirety. The new polymer membranes prepared from the aromatic polyimidemembranes by thermal treating and UV crosslinking that are selective tosulfur-containing molecules would be used to selectively removesulfur-containing molecules from fluid catalytic cracking (FCC) andother naphtha hydrocarbon streams. Further liquid phase examples includethe separation of one organic component from another organic component,e.g. to separate isomers of organic compounds. Mixtures of organiccompounds which may be separated using the new polymer membranesprepared from the aromatic polyimide membranes by thermal treating andUV crosslinking include: ethylacetate-ethanol, diethylether-ethanol,acetic acid-ethanol, benzene-ethanol, chloroform-ethanol,chloroform-methanol, acetone-isopropylether, allylalcohol-allylether,allylalcohol-cyclohexane, butanol-butylacetate, butanol-1-butylether,ethanol-ethylbutylether, propylacetate-propanol,isopropylether-isopropanol, methanol-ethanol-isopropanol, andethylacetate-ethanol-acetic acid.

The new polymer membranes prepared from the aromatic polyimide membranesby thermal treating and UV crosslinking may be used for separation oforganic molecules from water (e.g. ethanol and/or phenol from water bypervaporation) and removal of metal and other organic compounds fromwater.

The new polymer membranes prepared from the aromatic polyimide membranesby thermal treating and UV crosslinking described in the currentinvention have immediate applications for the separation of gas mixturesincluding carbon dioxide removal from natural gas. The new polymermembrane permits carbon dioxide to diffuse through at a faster rate thanthe methane in the natural gas. Carbon dioxide has a higher permeationrate than methane because of higher solubility, higher diffusivity, orboth. Thus, carbon dioxide enriches on the permeate side of themembrane, and methane enriches on the feed (or reject) side of themembrane.

An additional application of the new polymer membranes prepared from thearomatic polyimide membranes by thermal treating and UV crosslinking isas the separator in chemical reactors to enhance the yield ofequilibrium-limited reactions by selective removal of a specificbyproduct or product.

Yet another application of the new polymer membranes prepared from thearomatic polyimide membranes by thermal treating and UV crosslinkingdescribed in the current invention is as the catalytic polymericmembranes by loading metal catalysts or polymer-anchored metalcatalysts, or molecular sieve catalysts to the new polymer membranesprepared from the aromatic polyimide membranes by thermal treating andUV crosslinking. These new polymer membranes prepared from the aromaticpolyimide membranes by thermal treating and UV crosslinking are suitablefor a variety of catalysis applications that would be of interest in thecatalysis industry such as selective hydrogenation reactions to removefeed or product impurities, solid acid motor fuel alkylation (alkylene),ethylbenzene and cumene alkylation, detergent alkylation, C₃-C₅ lightolefin oligomerization, disproportionation and transalkylation processesto convert toluene to benzene and xylenes, selective conversion of ethylbenzene to paraxylene isomer, and others known to those of ordinaryskill in the art. The control of adsorption and diffusion properties bytailoring the characteristics of both the new polymer membranes preparedfrom the aromatic polyimide membranes by thermal treating and UVcrosslinking and catalyst components can greatly improve processefficiency that can only be achieved in systems of liquid acids orbases, where great efficiency is achieved via great partition of onereactant relative to others or the reactants relative to product. Thenew polymer membranes prepared from the aromatic polyimide membranes bythermal treating and UV crosslinking described in the current inventionpossess many advantages over traditional catalysts for theabove-mentioned catalysis applications.

As an example, the advantages of these new polymer membranes preparedfrom the aromatic polyimide membranes by thermal treating and UVcrosslinking for selective hydrogenation reactions such as selectivehydrogenation of propadiene and propyne and selective hydrogenation ofbutadiene include: 1) taking advantage of the catalytic membrane reactorconcept by combining chemical reactions with the catalytic andseparation activities of the membranes; 2) controllable H₂concentration; 3) adjustable H₂/feed ratio, etc. These advancedcharacteristics will improve the reaction yield and selectivitysimultaneously for selective hydrogenation reactions.

Yet another application of the new polymer membranes prepared from thearomatic polyimide membranes by thermal treating and UV crosslinkingdescribed in the current invention is as the novel efficientproton-conducting membrane for fuel cell application. The development ofefficient proton-conducting membrane is of the greatest importance forthe design and improvement of low-temperature fuel cells includingproton exchange membrane fuel cells (PEMFCs) and direct methanol fuelcells (DMFCs). PEMFC is one of the most attractive power sources for avariety of applications by virtue of its high efficiency and environmentfriendly nature. During the past two decades most of the activity in thefield of proton-conducting membranes has been undertaken by thematerials science community whose major motivation has been to developsuitable proton conducting materials for application as protonconducting membranes for fuel cells. The breakthrough of the PEMFC andDMFC technologies has been however still inhibited, mainly due to thelack of suitable materials for proton-conducting membrane applications.Optimized proton and water transport properties of the membrane arecrucial for efficient fuel cell operation. Dehydration of the membranereduces proton conductivity while excess of water can lead to floodingof the electrodes, both conditions may result in poor cell performance.

The new polymer membranes prepared from the aromatic polyimide membranesby thermal treating and UV crosslinking described in the currentinvention are expected to exhibit significantly improved performance asproton-conducting membranes for fuel cell applications compared totraditional Nafion® polymer membranes because of their excellent protonconducting property, high water adsorption capacity, and high chemicaland thermal stability.

In summary, the high performance new polymer membranes prepared from thearomatic polyimide membranes by thermal treating and UV crosslinking ofthe present invention are not only suitable for a variety of liquid,gas, and vapor separations such as desalination of water by reverseosmosis, non-aqueous liquid separation such as deep desulfurization ofgasoline and diesel fuels, ethanol/water separations, pervaporationdehydration of aqueous/organic mixtures, CO₂/CH₄, CO₂/N₂, H₂/CH₄, O₂/N₂,H₂S/CH₄, olefin/paraffin, iso/normal paraffins separations, and otherlight gas mixture separations, but also can be used for otherapplications such as for catalysis and fuel cell applications.

EXAMPLES

The following examples are provided to illustrate one or more preferredembodiments of the invention, but are not limited embodiments thereof.Numerous variations can be made to the following examples that liewithin the scope of the invention.

Example 1 Synthesis of poly(BTDA-APAF)polyimide

An aromatic poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(BTDA-APAF)) polyimide containing UV cross-linkable carbonyl groupsand pendent —OH functional groups ortho to the heterocyclic imidenitrogen in the polymer backbone was synthesized from2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane diamine (BTDA) and3,3′,4,4′-benzophenonetetracarboxylic dianhydride (APAF) in NMP polarsolvent by a two-step process involving the formation of the poly(amicacid) followed by a solution imidization process. Acetic anhydride wasused as the dehydrating agent and pyridine was used as the imidizationcatalyst for the solution imidization reaction. For example, a 250 mLthree-neck round-bottom flask equipped with a nitrogen inlet and amechanical stirrer was charged with 10.0 g (27.3 mmol) of APAF and 40 mLof NMP. Once the APAF was fully dissolved, a solution of BTDA (8.8 g,27.3 mmol) in 40 mL of NMP was added to the APAF solution in the flask.The reaction mixture was mechanically stirred for 24 hours at ambienttemperature to give a viscous poly(amic acid) solution. Then 11.1 g ofacetic anhydride in 10 mL of NMP was added slowly to the reactionmixture under stirring followed by the addition of 8.6 g of pyridine in10 mL of NMP to the reaction mixture. The reaction mixture wasmechanically stirred for an additional 1 hour at 80° C. to yield thepoly(BTDA-APAF) polyimide. The poly(BTDA-APAF) polyimide product in afine fiber form was recovered by slowly precipitating the reactionmixture into a large amount of methanol. The resultant poly(BTDA-APAF)polyimide fibers were then thoroughly rinsed with methanol and dried ina vacuum oven at 150° C. for 24 hours.

Example 2 Preparation of poly(BTDA-APAF) polyimide polymer membrane

The poly(BTDA-APAF) polyimide polymer membrane was prepared as follows:4.0 g of poly(BTDA-APAF) polyimide synthesized in Example 1 wasdissolved in a solvent mixture of 12.0 g of NMP and 12.0 g of1,3-dioxolane. The mixture was mechanically stirred for 2 h to form ahomogeneous casting dope. The resulting homogeneous casting dope wasallowed to degas overnight. The poly(BTDA-APAF) polymer membrane wasprepared from the bubble free casting dope on a clean glass plate usinga doctor knife with a 20-mil gap. The membrane together with the glassplate was then put into a vacuum oven. The solvents were removed byslowly increasing the vacuum and the temperature of the vacuum oven.Finally, the membrane was dried at 200° C. under vacuum for at least 48hours to completely remove the residual solvents to form poly(BTDA-APAF)polymer membrane (abbreviated as BTDA-APAF membrane).

Example 3 Preparation of New Polymer Membrane from BTDA-APAF Membrane byThermal Treatment and UV Crosslinking

The BTDA-APAF membrane prepared in Example 2 was thermally heated from500 to 450° C. at a heating rate of 5° C./min under N₂ flow. Themembrane was hold for 1 h at 450° C. and then cooled down to 50° C. at aheating rate of 5° C./min under N₂ flow. The heat-treated BTDA-APAFmembrane was then exposed to UV radiation using 254 nm wavelength UVlight generated from a UV lamp with 1.9 cm (0.75 inch) distance from themembrane surface to the UV lamp and a radiation time of 20 minutes at50° C. The UV lamp that was used was a low pressure, mercury arcimmersion UV quartz 12 watt lamp with 12 watt power supply from AceGlass Incorporated. The heat-treated and then UV crosslinked newmembrane was abbreviated as BTDA-APAF-HT-UV membrane.

Example 4 CO₂/CH₄ Separation Performance of BTDA-APAF Membrane andBTDA-APAF-HT-UV Membrane

The BTDA-APAF membrane and the BTDA-APAF-HT-UV membrane were tested forCO₂/CH₄ separation under testing temperature of 50° C. (Table 1). TheBTDA-APAF-HT-UV membrane was also tested at 100° C. It can be seen fromTable 1 that the BTDA-APAF-HT-UV membrane showed significantly increasedCO₂/CH₄ selectivity and CO₂ permeability compared to the untreatedBTDA-APAF membrane at 50° C. testing temperature. In addition, theBTDA-APAF-HT-UV membrane also showed good performance at 100° C. hightesting temperature. These results suggest that the BTDA-APAF-HT-UVmembrane is a good candidate for CO₂/CH₄ separation.

TABLE 1 Pure gas permeation test results of BTDA-APAF membrane andBTDA-APAF-HT-UV membrane for CO₂/CH₄ separation Membrane P_(CO2)(Barrer) α_(CO2/CH4) BTDA-APAF membrane^(a) 5.92 32.5BTDA-APAF-HT-UV^(a) 219.5 48.4 BTDA-APAF-HT-UV^(b) 325.3 19.7^(a)P_(CO2) and P_(CH4) were tested at 50° C. and 690 kPa (100 psig);^(b)P_(CO2) and P_(CH4) were tested at 100° C. and 690 kPa (100 psig); 1Barrer = 10⁻¹⁰ cm³ (STP) · cm/cm² · sec · cmHg.

Example 5 H₂/CH₄ Separation Performance of BTDA-APAF Membrane andBTDA-APAF-HT-UV Membranes

The BTDA-APAF membrane and the BTDA-APAF-HT-UV membrane were tested forH₂/CH₄ separation under testing temperatures of 50° C. (Table 2). It canbe seen from Table 2 that the BTDA-APAF-HT-UV membrane showedsignificantly increased H₂ permeability and maintained H₂/CH₄selectivity compared to the untreated BTDA-APAF membrane. These resultssuggest that the BTDA-APAF-HT-UV membrane is a good candidate for H₂/CH₄separation.

TABLE 2 Pure gas permeation test results of BTDA-APAF membrane andBTDA-APAF-HT-UV membrane for H₂/CH₄ separation^(a) Membrane P_(H2)(Barrer) α_(H2/CH4) BTDA-APAF membrane 24.6 134.9 BTDA-APAF-HT-UV 604.7133.2 ^(a)P_(H2) and P_(CH4) were tested at 50° C. and 690 kPa (100psig); 1 Barrer = 10⁻¹⁰ cm³ (STP) · cm/cm² · sec · cmHg.

Comparable Examples

Five process simulation examples were studied to compare the new highperformance BTDA-APAF-HT-UV membrane with the commercially availablemembranes. Comparable Example 1 was a single stage system using thecurrently commercially available membranes. Comparable Examples 2 and 3were a single stage system using the new BTDA-APAF-HT-UV membrane listedin Table 1. Comparable Example 1 and Example 2 were operated at feedtemperature of 50° C. In order to have a sufficient dew point marginpreventing liquid condensation on the membrane surface during theoperation, a pretreatment regenerable adsorbent system called MemGuard™that uses molecular sieves developed by UOP, was applied in these twoexamples. Comparable Example 3 was operated at high feed temperature of100° C. due to the high thermal and mechanical stability of the newBTDA-APAF-HT-UV membrane. Since sufficient dew point margin was providedby operating the membrane system at the high temperature, nopretreatment system was required in Comparable Example 3.

In order to improve the recovery of hydrocarbons from the natural gasstream, a two-stage membrane system was studied. In Comparable Example4, commercially available membranes were used for both first and secondstages. A pretreatment system such as MemGuard™ would be required forComparable Example 4. In Comparable Example 5, new BTDA-APAF-HT-UVmembrane was used for both first- and second-stage membranes. ComparableExample 5 operated the first stage at an elevated temeprature to providea sufficient dew point margin for the product gas. No pretreatmentsystem was required for Comparable Example 5. The second stage ofComparable Example 5 was operated at 50° C. feed temperature to increasethe membrane selectivity, hence, reduce the hydrocarbon loss. Sinceheavy hydrocarbons are hard to reach second stage feed, the pretreatmentunit such as MemGuard™ was not required.

Comparable Examples 1, 2, and 3 assumed a natural gas feed with 8% CO₂,and the product spec for CO₂ is at 2%. In Comparable Example 1, thecommercially available membrane was assumed to be a membrane withtypical performance in the current natural gas upgrading market. InComparable Examples 2 and 3, the new BTDA-APAF-HT-UV membrane materialwas used to make the membrane with a thickness of 200 nm. The permeanceof the new BTDA-APAF-HT-UV membrane was assumed at 0.030m³(STP)/m².h.kPa at 50° C. and 0.044 m³(STP)/m².h.kPa at 100° C. basedon the permeability measured for the dense membrane, and theselectivities were assumed at 44 at 50° C. and 15 at 100° C., which arelower than the selectivities shown in Table 1. A process simulationbased on the above performance was performed for Comparable Examples 1,2 and 3. The results are shown in Table 3.

TABLE 3 Simulation Results for Comparable Examples 1, 2 and 3 ComparableComparable Comparable Example 1 Example 2 Example 3 Feed Flow, m³(STP)/h5.9 × 10⁵ 5.9 × 10⁵ 5.9 × 10⁵ CO2 in Feed, % 8 8 8 CO2 in ProductRequired, % 2 2 2 MemGuard ™ Required? Yes Yes No Membrane FeedTemperature, 50 50 100 ° C. Membrane Feed Pressure, kPa 3792.3 3792.33792.3 Membrane Area Saved, % — 59.8 82.6 Total Hydrocarbon Recovery,Base 7.4 −2.8 %

It can be seen by comparing the above examples that Comparable Example 2showed significant cost saving (59.8% less membrane area required) andhigher hydrocarbon recovery (7.4% more) compared to ComparableExample 1. Comparable Example 3 not only can save the membrane area(82.6%), but also can eliminate the costly MemGuard™ pretreatment systemat slightly lower hydrocarbon recovery. It is anticipated that the newBTDA-APAF-HT-UV membrane system will significantly reduce the membranesystem cost and footprint which is extremely important for largeoffshore gas processing projects.

The hydrocarbon recovery can be increased by running a two stagemembrane system as shown in Comparable Examples 4 and 5. In ComparableExample 4, both stages applied the commercially available membranes withthe performance data the same as those in Comparable Example 1. InComparable Example 4, the new BTDA-APAF-HT-UV membrane was used for bothfirst stage and second stage. The first stage was operated at elevatedtemperature to eliminate the MemGuard™ system. The second stage wasoperated at lower temperature to increase the selectivity. The naturalgas feed in Comparable Examples 3 and 4 had been changed to 45% CO₂(more meaningful for a two-stage system), and the product specificationfor CO₂ in these two examples were assumed at 8%. Table 4 shows theresults of the simulation for Comparable Examples 4 and 5.

TABLE 4 Simulation Results for Comparable Examples 4 and 5 ComparableComparable Example 4 Example 5 Feed Flow, m³(STP)/h 5.9 × 10⁵ 5.9 × 10⁵CO₂ in Feed, % 45 45 CO₂ in Product Required, % 8 8 Pretreatmentrequired? Yes No 1^(st) Stage Membrane Feed Temperature, ° C. 50 1001^(st) Stage Membrane Feed Pressure, kPa 3792.3 3792.3 2^(nd) StageMembrane Feed Temperature, ° C. 50 50 2nd Stage Membrane Feed Pressure,kPa 3902.6 3902.6 1^(st) Stage Membrane area Base 20.5% 2^(nd) StageMembrane area Base 40.8% Total Compressor Horse Power Base 107.5% TotalHydrocarbon Recovery, % 96.9 97.1

It can be seen from Table 4 that Comparable Example 4 and ComparableExample 5 have very similar hydrocarbon recovery. Due to the hightemperature operation for the first stage membrane, Comparable Example 5does not require a pretreatment such as a MemGuard™ system, which isabout 10 to 40% of the total cost of Comparable Example 4. At the sametime, the first stage membrane area is reduced by 79.5% and the secondstage membrane area is reduced by 59.2% from Comparable Example 4 toComparable Example 5. It can be expected that the Comparable Example 5will have a big capital (>50%) and footprint (>50%) saving compared toComparable Example 4. The only drawback is that the compressor will beslightly bigger. Table 4 shows a 7.5% horse power increase fromComparable Example 4 to Comparable Example 5.

1. A polymer membrane made from subjecting aromatic polyimide membranesfirst to a thermal treatment step followed by a crosslinking stepwherein said aromatic polyimide membranes comprise at least one aromaticpolyimide polymer comprising cross-linkable functional groups in apolymer backbone of said aromatic polyimide polymer and pendent hydroxylfunctional groups ortho to a heterocyclic imide nitrogen within saidaromatic polyimide polymer.
 2. The polymer membrane of claim 1 whereinsaid crosslinking step is a chemical crosslinking step or an exposure toUV radiation to cause crosslinking of said aromatic polyimide polymer.3. The polymer membrane of claim 1 wherein said aromatic polyimidepolymer has a structure represented by a plurality of first repeatingunits of a formula (I), wherein said formula (I) is:

where —X₁— of said formula (I) is

or mixtures thereof, —X₂— of said formula (I) is either the same as —X₁—or is selected from

or mixtures thereof, —X₃— of said formula (I) is

or mixtures thereof, -Z-, -Z′-, and -Z″- are independently —O— or —S—,and —R— is

or mixtures thereof.
 4. The polymer membranes of claim 1 wherein saidmembranes have a structure comprising a nonporous symmetric structure oran asymmetric structure having a thin nonporous dense selective layer ontop of a porous support layer.
 5. The polymer membranes of claim 1fabricated into a geometry selected from flat sheet, spiral wound, disk,tube, hollow fiber or thin film composite.
 6. The polymer membranes ofclaim 1 wherein said polymer membranes are catalytic membranes used in acatalysis application.
 7. The polymer membranes of claim 6 wherein saidcatalytic membranes are used in a process selected from the groupconsisting of removal of feed or product impurities, hydrogenationreactions, solid acid motor fuel alkylation, ethyl benzene and cumenealkylation, detergent alkylation, C₃-C₅ light olefin oligomerization,conversion of toluene and C₉ and C₁₀ aromatics to benzenes and xylenes,and selective conversion of ethyl benzene to paraxylene isomers. 8 Thepolymer membranes of claim 1 wherein said membranes are used indesulphurization of gasoline or diesel fuels.
 9. The polymer membranesof claim 1 wherein said membranes are used in separation of liquidmixtures by pervaporation.
 10. The polymer membranes of claim 1 whereinsaid membranes are used in separation of liquid mixtures selected fromthe group of pairs of liquids consisting of ethylacetate-ethanol,diethylether-ethanol, acetic acid-ethanol, benzene-ethanol,chloroform-ethanol, chloroform-methanol, acetone-isopropylether,allylalcohol-allylether, allylalcohol-cyclohexane, butanol-butylacetate,butanol-1-butylether, ethanol-ethylbutylether, propylacetate-propanol,isopropylether-isopropanol, methanol-ethanol-isopropanol, andethylacetate-ethanol-acetic acid.
 11. The polymer membranes of claim 1wherein said membranes are used in separation of organic molecules fromwater or removal of metal and other organic molecules from water.
 12. Amethod for making polymer membranes from aromatic polyimide membranes bythermal treating and UV crosslinking comprising: a) first providing orsynthesizing a polyimide polymer wherein said polyimide polymer containspendent —OH or —SH groups ortho to a heterocyclic imide nitrogen and UVcross-linkable functional groups in a polymer backbone; b) fabricatingpolyimide membranes from the polyimide polymer; c) heating the polyimidemembranes at a temperature between about 300° and 600° C. under inertatmosphere or vacuum for about 30 seconds to one hour to produce aheated polyimide membrane; and d) exposing said heated polyimidemembranes to a crosslinking treatment.
 13. The method of claim 12wherein said crosslinking treatment is selected from the groupconsisting of chemical crosslinking and UV radiation.
 14. The method ofclaim 12 further comprising coating a top surface of the polymermembranes made from aromatic polyimide membranes by thermal treating andUV crosslinking with a thin layer of high permeability material selectedfrom the group consisting of a polysiloxane, a fluoro-polymer, athermally curable silicone rubber, and a UV radiation curable epoxysilicone.
 15. The method of claim 12 wherein said polyimide polymercomprises a plurality of repeating units of a formula (I), wherein saidformula (I) is:

where —X₁— of said formula (I) is

or mixtures thereof, —X₂— of said formula (I) is either the same as —X₁—or selected from

or mixtures thereof, —Y— of said formula (I) is

or mixtures thereof, -Z-, -Z′-, and -Z″- are independently —O— or —S—,—R— is

or mixtures thereof.
 16. The method of claim 12 wherein said polyimidepolymer is selected from the group consisting ofpoly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropanel(poly(BTDA-APAF)), poly(3,3′,4,4′-benzophenonetetracarboxylicdianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl) (poly(BTDA-HAB)),poly[3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(DSDA-APAF)), poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl)(poly(DSDA-APAF-HAB)),poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(6FDA-BTDA-APAF)), poly[4,4′-oxydiphthalicanhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(ODPA-APAF)), poly[4,4′-oxydiphthalicanhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl](poly(ODPA-APAF-HAB)), poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl](poly(BTDA-APAF-HAB)), and poly(4,4′-bisphenol Adianhydride-3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane](poly(BPADA-BTDA-APAF)).
 17. A process for separating at least one gasor liquid from a mixture of gases or liquids using a polymer membraneprepared from an aromatic polyimide membrane by thermal treating and UVcrosslinking, said process comprising: a) providing a polymer membraneprepared from polyimide polymers comprising cross-linkable functionalgroups found in a backbone of said cross-linkable polyimide polymers andpendent —OH or —SH groups ortho to a heterocyclic imide nitrogen whereinsaid polyimide polymers are first subjected to a heat treatment toproduce a heat treated polyimide polymer membrane and then said heattreated polyimide polymer membrane is subjected to a crosslinkingtreatment wherein said polymer membrane prepared from the aromaticpolyimide membrane by thermal treating and UV crosslinking is permeableto at least one gas or liquid; b) contacting a mixture of gases orliquids on one side of the polymer membrane prepared from the aromaticpolyimide membrane by thermal treating and UV crosslinking to cause atleast one gas or liquid to permeate the membrane; and c) removing froman opposite side of the membrane a permeate gas or liquid compositionthat is a portion of said at least one gas or liquid that permeated themembrane.
 18. The process of claim 17 wherein said gases are mixturesselected from the group consisting of CO₂/CH₄, CO₂/N₂, H₂/CH₄, O₂/N₂,H₂S/CH₄, olefin/paraffin, and iso paraffins/normal paraffins.
 19. Theprocess of claim 17 wherein said gas or liquid comprises at least onevolatile organic compound in an atmospheric gas.
 20. The process ofclaim 17 wherein said gases or liquids comprise hydrogen from ahydrocarbon vapor stream.
 21. The process of claim 17 wherein said gasesor liquids comprise a mixture of at least one pair of gases selectedfrom the group consisting of nitrogen and oxygen, carbon dioxide andmethane, and hydrogen and methane or a mixture of carbon monoxide,helium and methane.
 22. The process of claim 17 wherein said gases orliquids comprise natural gas comprising methane and at least one gascomponent selected from the group consisting of carbon dioxide, oxygen,nitrogen, water vapor, hydrogen sulfide, helium and other trace gases.23. The process of claim 17 wherein said gases or liquids comprisehydrocarbon gases, carbon dioxide, hydrogen sulfide and mixturesthereof.